With the trend to continue to miniaturize semiconductor integrated circuits to achieve submicron feature sizes, photolithography has become one of the most critical steps in semiconductor manufacturing. The goal of photolithography in establishing the critical dimensions of the various devices and circuits is to create a pattern which meets design requirements as well as to correctly align the circuit pattern on the surface of the wafer.
As line widths shrink smaller and smaller in submicron photolithography, the process of printing lines and contact holes in photoresist becomes increasingly more difficult. Photoresists have been developed to keep pace with the industry's need to print narrower lines with fewer defects. The selection of the photoresist must be made on whether the photoresist has the capability of producing the design dimensions. A thinner resist layer will generally improve the capability of printing smaller feature sizes. However, the resist must simultaneously be thick enough to act as an etchant barrier and be free of pinholes.
The smallest equal lines and spaces that can be formed in the photoresist layer is known as the resolution capability. Thinner photoresist film thicknesses will improve the resolution capability.
As circuits increase in the number of layers, the wafer surface becomes less planar. The resolution of small image sizes is further reduced due to the difference in the topography areas in which the photoresist thickness becomes uneven across the wafer surface. When the exposure radiation is directed at the wafer surface at a 90.degree. angle to the surface, a well-defined image is created in the photoresist because the exposing waves reflect up and down in the resist. If, however, any of the exposing radiation waves reflect at angles other than 90.degree. up from the surface beneath the photoresist, unwanted portions of the photoresist will be exposed. Variations in the subsurface topography intensify the problem of reflection. The sidewalls of the steps, for example, reflect the radiation at various angles, causing poor image resolution. A smooth surface under the photoresist will eliminate much of the reflection problems.
In addition, where the surface beneath the photoresist is uneven, the exposure time of the photoresist must be sufficient to expose the photoresist at its thickest depth. This means that the thinner areas of photoresist will be over exposed, resulting in wider lateral openings or narrower lines which will produce poorer image resolution.
It would be desirable to provide a fabrication technique having a planarized surface beneath the photoresist layer to achieve better image resolution and smaller image-size openings. It would further be desirable for such technique to use a thin photoresist to achieve smaller line openings. It would also be desirable for such fabrication technique to be easily adapted for use with standard integrated circuit process flows.